As known in the art, in accordance with the distributed temperature sensing (DTS) technique, temperatures are recorded along a sensing optical fiber positioned in a long length area of interest as a continuous profile, wherein the area length could be greater than 30 km. In a typical time-domain DTS device pulsed optical radiation from a light source is directed into the sensing optical fiber, which leads to spontaneous Raman back-scattering. Stokes components of the Raman back-scattering have a lower frequency than the optical radiation from the light source, wherein Anti-Stokes components have a higher frequency than the optical radiation from the light source. The back-scattered optical radiation is analyzed to obtain a temperature profile of the area of interest. Since the intensity of anti-Stokes back-scattered optical radiation is highly dependent on temperature, the temperature profile is derived based on a ratio between the intensities of anti-Stokes Raman and Stokes Raman backscattered components.
A major problem inherent to known DTS measurement systems is providing high accuracy of temperature determination. Known solutions for providing high accuracy temperature measurements include adjusting the attenuation profile of the Stokes component to that of the anti-Stokes component to compensate the difference in the attenuation profiles of these components. This adjustment is made in the assumption of the components having a smooth exponential attenuation profile, which may not always be a reality.
Other solutions use two additional optical radiation sources: one in the Stokes frequency of the primary optical source and the other in the anti-Stokes frequency of the primary optical source. As a result, Rayleigh backscattered optical radiation components are generated on the Stokes and Anti-Stokes frequency, which are used for correction of the attenuation profile of the back-scattered Stokes and anti-Stokes optical radiation. However, this approach brings additional cost and complexity to the device, the latter being especially challenging in field applications.
Another known solution is to use a reference optical fiber with predetermined optical properties connected to the sensing optical fiber, which allows for self calibrating of the measurement device and hence more accurate measuring of temperature profiles. This solution does not however take care of additional attenuation alteration which may occur due to various environmental changes.
Thus a need still exists for a more cost-effective and more convenient in field applications high precision fiber-optic system and method for measuring temperature profiles in long length areas.